I'm still thinking about / working out details. The core concepts that @krl proposed seemed to be so unexplored so I wanted to just put it out there for everyone. That's why I was a bit sketchy in some answers, I was thinking about the concepts while writing, improvising.
Maybe meet-up groups and meet-up locations could be generated and handed out under some sort of encryption, so that they're only known to the peers who meet one another. That feels technically possible. And if IDs are renewed with no connection to the previous months IDs, that makes the tokens almost completely anonymous. And, if the locations that each peer registers could be encrypted too, then that makes it completely anonymous. This could be done through homomorphic encryption or something. I'm not an expert at that technical stuff and i'll have to ask around and google around a bit.
I came up with a financial incentive thing yesterday, https://www.reddit.com/r/ethereum/comments/3io193/proofofindividuality_poi_id_new_financial/
That it excludes a few is the biggest problem. But there's been similar events in history where everyone left what they had at hand and joined some cause. Like, town meetings, sabbath, I'm not sure if the whole global meet-up event would be as problematic as it might sound.
I've been working a lot with basic income. I paused / dropped out of my medical studies in 2010 because I felt like society was making people sick, and I started blogging about UBI in 2012 and was somewhat influential, i'm quoted by the BIG movement that became basicincome2013.eu. I designed a new type of basic income system that gained little attention back then in 2013 but that has [gained more attention now in 2014-2015](www.basicincome.org/news/2015/04/bitnation-basic-income-application-set-for-bitnation/). My sort of main argument for it is that it decreases the prevalence of fight and flight symptoms, as people get to choose and control their own lives. Agorophobia and the concept schizophrenia are fight and flight symptoms and I don't think they'll exist in basic income societies. Narcolepsy doesn't exclude someone from going to a meet-up. Transportation has not excluded people from attending other compulsory stuff in the past. Not having a computer isn't really a real issue to me, like, crypto UBI pretty much assumes crypto-currency users will have access to the web.
The real issue is that it excludes those who absolutely cannot meet-up with their groups. Surgeons performing acute surgery. But, meet-ups could be held at hospitals too, so that could make it possible for most people at hospitals to go to meet-ups. Like, people have to meet strangers in hospitals all the time even tough it sucks. When I was a medical student, people were forced to meet me even tough they hadn't paid for my service. Medical students invade everyone's privacy all the time. So, if like a few people came and visited me when I was in the hospital and we verified each other as humans and it took like an hour, maybe that's okay. But these are problems, and I agree with u. I'm just not sure if they're that big problems, it's all feedback loops. And with Watson the super-computer diagnosing diseases and lab-on-a-chip technology in our smartphones, maybe we won't be spending too much time in hospitals anyway. And planned surgery could easily be scheduled so that everyone has time to go to a meet-up.
These are issues that need to be discussed. But prior to that, we should talk about if the underlying concepts work and in what ways they're better or worse then other ID systems. Web-of-trust IDs have all sorts of problems too, so, what's the least worst ID ?